Abstract: The water exchange capacity is only related to the fluidity of water.At present, most of the researches on water exchange rely on the concentration of indicator material to calculate water exchange rate and exchange time.But the results cannot reflect the exchange capacity of seawater accurately.Taking a T-shaped region as an example, the paper proposes a calculation method of water exchange based on the flow function.First, obtaining a two-dimensional shallow water equation with the average depth of water based on the integral of the three-dimensional continuous equation along the water depth.By solving the equation, data such as the velocity and depth are obtained.Then making contour map corresponding to flow function, which is obtained by integrating the two-dimensional shallow water equation.It can be clearly seen from the contour map that more than half of the water in the simulated area can be exchanged out of the area.Respectively, it is calculated that in the front and back half of the model the seawater exchange rates are 0.6396 and 0.6465.The result shows that it is simple and feasible to evaluate the water exchange capacity of a certain area by using the numerical simulation method based on the flow function.The contour map can directly reflect the water exchange capacity.